If an air-fuel ratio of an air-fuel mixture in a combustion chamber of an internal combustion engine is referred as an engine air-fuel ratio, and if a ratio of the total amount of air fed into the intake passage, the combustion chamber, and the exhaust passage upstream of a certain position in the exhaust passage to the total amount of fuel fed into the intake passage, the combustion chamber, and the exhaust passage upstream of the above-mentioned position is referred to as an exhaust gas air-fuel ratio of the exhaust gas flowing through the certain position, the Japanese Unexamined Patent Publication No. 4-365920 discloses an exhaust gas purifying device for an internal combustion engine with multi-cylinders, the engine having first and second cylinder groups, in which the device is provided with: an engine operation control device to make each cylinder of the first cylinder group a rich engine operation in which the engine air-fuel ratio is rich, and to make each cylinder of the second cylinder group a lean engine operation in which the engine air-fuel ratio is lean; a first exhaust passage connected to each cylinder of the first cylinder group; a second exhaust passage connected to each cylinder of the second cylinder group and different from the first exhaust passage; an NH.sub.3 synthesizing catalyst arranged in the first exhaust passage for synthesizing ammonia NH.sub.3 from at least a part of NO.sub.x in the inflowing exhaust gas; an interconnecting passage interconnecting the first exhaust passage downstream of the NH.sub.3 synthesizing catalyst and the second exhaust passage to each other; and an exhaust gas purifying catalyst arranged in the interconnecting passage to reduce NO.sub.x from the second exhaust passage by NH.sub.3 from the first exhaust passage.
In the above engine, the fuel consumption rate is reduced by increasing the numbers of the cylinders of the second cylinder group in which the lean engine operation is performed. However, if the numbers of the cylinders of the first group are decreased and the numbers of the cylinders of the second group are increased, the NH.sub.3 amount flowing into the exhaust gas purifying catalyst decreases and the NO.sub.x amount flowing into the catalyst increases. As a result, the NO.sub.x amount flowing into the catalyst may be excessive with respect to the NH.sub.3 amount, and thus NO.sub.x may be emitted from the catalyst without being reduced sufficiently.